The transmitter converts an electrical signal containing video and/or audio information into a signal radiation which is preferably located in the IR wavelength range. The electrical signal can be provided, for example, by an HF receiver for terrestrial or satellite television broadcast, a video recorder, an audio and/or video disc player or a video camera. In the photo receiver, an optical sensor converts the signal radiation back into the electrical signal form of the transmitter; the photo receiver is preferably connected to or housed in a playback unit for video and/or audio. Playback units are, for example, flat panel displays, video projectors, or audio systems which are located remote from the video playback unit. A conventional television set or a set top box can also be connected to a photo receiver.
Advances in television technology have led to improvements in the video and audio playback with the goal to approach the playback quality of large screen movies with multichannel audio recording. Television sets have also been combined with devices for other media, such as, for example, with a PC, a telephone network, and a video disc player for, for example, a CD-ROM, CD-i and Video-CD which will be referred to hereinafter as Video-CD.
This necessitates novel video and audio playback units, for example, flat panel displays which are advantageously located remote from the other devices.
The advantages of a flat panel display which can be hung on a wall, can hardly be exploited if this display is combined with a conventional television receiver to form a single unit to which a plurality of cables is connected.
Moreover, video disk players are generally designed to play several types of CD's, and provide playback signals for the display of a TV set as well as for an audio system. It is therefore desirable to employ wireless signal connections for transmitting video and audio information between these devices.
A method and a device for wireless optical transmission of audio information in the form of digital audio data to a wireless headset via IR radiation is known, for example, from DE-C2-43 28 252. The signal source is a digital audio source, such as a CD player, DAT recorder or a DSR tuner. The audio data are sampled values of stereo audio signals, each with a 16 bit word length, and reflect the digital source encoding used during recording. Before the data are transmitted with the IR radiation, an encoder converts the audio data into a serially channel-encoded data signal. Additional bits and data words are added to the data stream for synchronizing the receiver and for error correction. This requires a data transmission rate of about 2 Mbit/s.
The audio data are extracted from a serial digital audio interface according to the interface standard (DIN EN 60958, IEC 958). The sampled values are thus available in form of a Biphase-Mark-Code (BM-Code). An additional decoding step is required before conversion into the channel-coded data signal, which increases the complexity of the circuit. Disadvantageously, the additional encoding and decoding required in order to match the properties of an IR channel significantly increases the complexity of the additional circuitry on both the transmitter side and the receiver side, without improving transmission quality.
It is a further disadvantage that a significant amount of electrical power is required on the transmitter side for generating sufficient radiating power for a reliable transmission.
U.S. Pat. No. 5,053,882 describes the transmission of a composite FM signal containing video and audio information, wherein an analog laser video player is connected to a photo receiver through a glass fiber cable. In the laser video player, the sampled signal is converted by a laser diode into IR radiation, bypassing a customary FM demodulator, with the signal form of the IR radiation resembling the recorded signal on the Video-CD. The video signal and the audio signals are regenerated from the transmitted signal on the receiver side with the help of filters and FM demodulators. Circuitry known from the laser video player can advantageously be used for this purpose. This significantly reduces the cost for matching the FM signal to the glass fiber cable. U.S. Pat. No. 5,053,882, however, does not provide a feasible solution for wireless transmission of a television signal to spatially remote photo receivers.
Conventional luminescent diodes (IRED) are inexpensive in comparison to laser diodes and radiate IR radiation over a wide angle. This is particularly advantageous for the intended application. Disadvantageously, however, only special types of diodes can be modulated with frequencies of 10 MHz and above; in addition, these diodes emit only a small amount of radiation at high frequencies as a result of the amplitude-frequency characteristics.
Another problem is obstruction by people or by articles of furniture as well as unmodulated ambient light or ambient light modulated at low frequencies, which is commonly present in the transmission space and which is superimposed on the signal radiation as a spurious signal. Both have the effect that on the receiver side, only a small amount of radiation is available, which is difficult to process, especially over a large signal frequency range, when ambient light is present, requiring steep filters and equalizers. For this reason, conventional systems employ many luminescent diodes in parallel in order to improve the radiation yield. This arrangement, however, is limited at higher modulation frequencies due to the frequency dependent self-impedance of the diodes.